All semiconductor materials are characterized by an energy gap (EG) greater than zero. The energy gap is defined as the difference between the conduction-band edge (EC) and the valence-band edge (EV). A deep-level state is defined as a state having an energy level at least 0.05 EG above the valence-band edge and at least 0.05 EG below the conduction-band edge.
A deep-level state can arise from a bound state of a substitutional impurity, an antisite, a vacancy on a lattice-site, an interstitial impurity, a dislocation, or a complex comprised of two or more deep-centers. For decades, semiconductor device designers have viewed deep-level states as something to be avoided, as their presence can degrade the performance (e.g., gain, speed) of many devices. As a result, traditional semiconductor processing techniques almost always seek to eliminate or minimize deep-level states.
At present, devices based on deep-level transitions are not being pursued because of widely-held beliefs that:    (1) optical-selection rules forbid deep-level-to-conduction band transitions;    (2) direct optical transitions (involving no phonons) from a deep-level to either the conduction or the valence band are very weak;    (3) phonons are required for deep-level-to-conduction-band, deep-level-to-valence-band, or deep-level-to-deep-level transitions (and that such phonon-assisted optical transitions are too unreliable, too weak, too irreproducible, or too temperature-dependent for use in devices); and,    (4) deep-levels always represent nonradiative recombination centers, and are not useful for optical or electrical devices.
Notwithstanding these commonly-held beliefs, devices based on deep-level transitions offer the potential to meet several long-felt, yet unsatisfied, needs in the art. In optoelectronics, for example, achieving a high degree of integration of electronics with optical devices is a long-desired, yet elusive, goal. GaAs circuits have achieved a very high degree of integration of electronic devices, but conventional GaAs optical devices operate at the 0.85 μm wavelength. In contrast, conventional InGaAs optical devices operate at the fiber optics wavelength of 1.3–1.5 μm. However, InP circuits have not achieved a level of device integration which is anywhere near that of GaAs device integration. InP device integration is still near its infancy. At present, the lack of very large scale monolithic integration of InP circuits with InGaAs optical devices has meant that some parts of optical modules are actually put together by hand (or with flip-chip bonding), which is very expensive indeed. Development of optically-active deep-level devices (that operate at fiber optic wavelengths) offers the potential to break this integration bottleneck, and enable monolithic integration of electronic and optoelectronic components on a single GaAs substrate. This is potentially a multibillion dollar industry.
Another long-felt, but unfulfilled need, exists in the manufacture of inexpensive light-emitting diodes (“LEDs”). At present, GaP accounts for about half the world's production of compound semiconductor substrates. These GaP substrates are used as large (eight-inch) substrates for visible (red, green, and yellow) LEDs. Such LEDs are fabricated on a lattice-matched substrate, then transferred to a GaP substrate. (For example, a red InGaP LED is removed from its GaAs substrate by epitaxial lift-off, and then transferred to a GaP substrate.) But this process of epitaxial lift-off and transfer to a GaP substrate substantially increases cost and lowers yield. The ability to fabricate visible LEDs directly on a GaP substrate (by using deep-level states) would obviate the need to remove LEDs from their original substrate and transfer LEDs to the GaP substrate, thus solving a long-standing problem in the multi-billion dollar LED industry.
Deep-level devices also offer the potential for integration of fiber optic components with Si electronics. For example, a deep-level device operating at 1.5 μm on GaAs could be integrated with Si electronics using Motorola's GaAs-on-Si technology. Alternatively, deep-level devices on GaP substrates could be integrated with Si using, for example, Oak Ridge National Laboratory's semiconductor-on-oxide-on-semiconductor technology, since GaP and Si have the same lattice constant (5.45 angstroms). The ability to effectively integrate fiber optic components with Si electronics would have tremendous market potential. This would be a multi-trillion dollar industry.
The invention, as described below, addresses these and other needs.